U.S. patent number 6,266,369 [Application Number 09/093,879] was granted by the patent office on 2001-07-24 for mpeg encoding technique for encoding web pages.
This patent grant is currently assigned to WorldGate Service, Inc.. Invention is credited to Joseph Ellis Augenbraun, Bruce Plotnick, Chuanming Wang.
United States Patent |
6,266,369 |
Wang , et al. |
July 24, 2001 |
**Please see images for:
( Certificate of Correction ) ** |
MPEG encoding technique for encoding web pages
Abstract
A method and apparatus for digitally encoding video image data,
which is particularly suited for encoding Internet Web pages,
eliminates the need for performing time consuming, computationally
intensive motion vector searches by taking advantage of prior
knowledge regarding the Web page movement. In a first preferred
embodiment, a digital video encoder, such as an MPEG encoder,
employs Web page scrolling coordinates obtained from a browser
application to determine, through calculation instead of searching,
motion estimation for all the macroblocks of the present image
frame relative to the previous frame. In a second preferred
embodiment, the encoder receives information from the browser
application that indicates that certain portions of an image, such
as a Web page animation window, are continuously changing, and thus
should be encoded as an intra frame. The invention may be employed
in a cable television system which includes Internet access
capabilities that permit system users to download Web pages for
viewing on their televisions from a remote browser application
located in the cable system's headend.
Inventors: |
Wang; Chuanming (Jamison,
PA), Plotnick; Bruce (Jamison, PA), Augenbraun; Joseph
Ellis (Princeton, NJ) |
Assignee: |
WorldGate Service, Inc.
(Trevose, PA)
|
Family
ID: |
22241485 |
Appl.
No.: |
09/093,879 |
Filed: |
June 9, 1998 |
Current U.S.
Class: |
375/240;
709/247 |
Current CPC
Class: |
H04N
19/172 (20141101); H04N 19/107 (20141101); H04N
19/137 (20141101); H04N 19/162 (20141101); H04N
19/533 (20141101); H04N 21/4622 (20130101) |
Current International
Class: |
H04N
5/445 (20060101); H04N 007/12 () |
Field of
Search: |
;348/394,400,401,402,407,412,413,415,416,13,14,15 ;345/161,173
;358/105 ;375/240 ;709/246,247 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
ISO/IEC, "Information Technology--Generic Coding of Moving Pictures
and Associated Audio Information: Video," International Standard
ISO/IEC 13818-2, 1st ed., (1996)..
|
Primary Examiner: Britton; Howard
Assistant Examiner: Diep; Nhon T
Attorney, Agent or Firm: Jones, Tullar & Cooper PC
Claims
What is claimed is:
1. A method for encoding multiple frame image data comprising:
a) generating a sequence of image frames;
b) supplying said sequence of image frames to an encoder;
c) providing information to said encoder identifying a
characteristic of said sequence of image frames; and
d) encoding at least a portion of said at least one frame in said
sequence without employing a motion vector search al origin by
employing said characteristic identifying information and image
information relating to at least one previous image frame in said
sequence.
2. The method of claim 1, wherein said sequence of image frames is
generated by an Internet browser application, and comprises
sequential images of one or more Internet Web pages.
3. The method of claim 1, wherein said characteristic identifying
information comprises scrolling coordinate information.
4. The method of claim 3, wherein said step of encoding further
comprises:
1) calculating at least one motion vector for a frame to be encoded
using said scrolling coordinate information; and
2) encoding said frame to be encoded as an inter frame using said
at least one motion vector and said image information for a
previous frame.
5. The method of claim 4, wherein said characteristic information
further includes location information identifying the location of
at least one animation window in an image, and said step of
encoding further comprises encoding a portion of said frame
containing said animation window as an intra frame.
6. The method of claim 3, wherein each of said frames in said
sequence is comprised of a plurality of multiple pixel macroblocks,
and said step of encoding further comprises:
1) comparing pixel intensities in a first macroblock of a frame to
be encoded with pixel intensities of a corresponding macroblock at
a same location in a previous frame;
2) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding macroblock is below a
predetermined threshold, then setting a motion vector to zero for
said macroblock to be encoded, and, if any uncoded macroblocks
remain in said frame to be encoded, returning to step 1 for a next
macroblock in said frame;
3) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding macroblock is above a
predetermined threshold, then comparing said macroblock to be
encoded with a corresponding shifted macroblock in a previous frame
that is shifted in direction and offset in accordance with said
scrolling coordinate information;
4) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding shifted macroblock is below a
predetermined threshold, then employing said scrolling coordinate
information to calculate a motion vector for said macroblock to be
encoded, and, if any uncoded macroblocks remain in said frame to be
encoded, returning to step 1 for a next macroblock in said frame;
and
5) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding shifted macroblock is above a
predetermined threshold, then encoding said macroblock to be
encoded as an intra-macroblock, and, if any uncoded macroblocks
remain in said frame to be encoded, returning to step 1 for a next
macroblock in said frame.
7. The method of claim 6, wherein steps 3 and 4 are performed
before steps 1 and 2 for a next macroblock to be encoded if the
difference in pixel intensities between a present macroblock to be
encoded and said corresponding shifted macroblock is below a
predetermined threshold, and the next macroblock to be encoded is
adjacent said present macroblock to be encoded.
8. The method of claim 1, wherein said characteristic identifying
information comprises location information identifying the location
of at least one animation window in an image, and said step of
encoding further comprises encoding said frame to be encoded using
said location information.
9. The method of claim 8, wherein each of said frames in said
sequence is comprised of a plurality of multiple pixel macroblocks,
and said step of encoding further comprises:
1) employing said location information to identify one or more
macroblocks in a frame to be encoded which include at least a
portion that forms part of said at least one animation window;
2) encoding said one or more macroblocks as intra-macroblocks;
and
3) encoding any remaining macroblocks in said frame to be encoded
as forward predictive coded macroblocks, with a zero motion
vector.
10. The method of claim 8, wherein each of said frames in said
sequence is comprised of a plurality of multiple pixel macroblocks,
and said step of encoding further comprises:
1) employing said location information to identify one or more
macroblocks in a frame to be encoded which include at least a
portion that forms part of said at least one animation window;
2) encoding said one or more macroblocks by employing a motion
vector search algorithm; and
3) encoding any remaining macroblocks in said frame to be encoded
as forward predictive coded macroblocks, with a zero motion
vector.
11. The method of claim 1, wherein said encoding is performed using
a digital video encoding format.
12. A system for encoding multiple frame image data comprising:
a) an image generating application for generating a sequence of
image frames to be encoded, said application including information
identifying a characteristic of said sequence of image frames;
and
b) an encoder for receiving said sequence of image frames from said
application and encoding said frames, said encoder including means
for obtaining said information from said application identifying a
characteristic of said sequence of image frames, and encoding at
least a portion of at least one frame in said sequence without
employing a motion vector search algorithm by employing said
characteristic identifying information and image information
relating to at least one previous image frame in said sequence.
13. The system of claim 12, wherein said image generating
application comprises an Internet browser application, and said
sequence of image frames comprise sequential images of one or more
Internet Web pages.
14. The system of claim 13, wherein said encoder is a digital video
encoder.
15. The system of claim 13, wherein said browser application and
said encoder are disposed in a cable television system headend, and
a plurality of downstream communication channels is provided for
transmitting encoded Internet Web page data from said encoder to
one or more remotely located set top converter boxes.
16. The system of claim 12, wherein said characteristic identifying
information comprises scrolling coordinate information which is
generated by said image generating application, and said encoder
includes means for generating motion vectors using said scrolling
coordinate information.
17. The system of claim 16, wherein said characteristic identifying
information further comprises location information identifying the
location of one or more animation windows in an image frame.
18. The system of claim 16, wherein each of said frames in said
sequence is comprised of a plurality of multiple pixel macroblocks,
and said encoder further includes means for carrying out the
following steps:
1) comparing pixel intensities in a first macroblock of a frame to
be encoded with pixel intensities of a corresponding macroblock at
a same location in a previous frame;
2) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding macroblock is below a
predetermined threshold, then setting a motion vector to zero for
said macroblock to be encoded, and, if any uncoded macroblocks
remain in said frame to be encoded, returning to step 1 for a next
macroblock in said frame;
3) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding macroblock is above a
predetermined threshold, then comparing said macroblock to be
encoded with a corresponding shifted macroblock in a previous frame
that is shifted in direction and offset in accordance with said
scrolling coordinate information;
4) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding shifted macroblock is below a
predetermined threshold, then employing said scrolling coordinate
information to calculate a motion vector for said macroblock to be
encoded, and, if any uncoded macroblocks remain in said frame to be
encoded, returning to step 1 for a next macroblock in said frame;
and
5) if the difference in pixel intensities between said macroblock
to be encoded and said corresponding shifted macroblock is above a
predetermined threshold, then encoding said macroblock to be
encoded as an intra-macroblock, and, if any uncoded macroblocks
remain in said frame to be encoded, returning to step 1 for a next
macroblock in said frame.
19. The system of claim 18, wherein said encoder further includes
means for performing steps 3 and 4 before steps 1 and 2 for a next
macroblock to be encoded if the difference in pixel intensities
between a present macroblock to be encoded and said corresponding
shifted macroblock is below a predetermined threshold, and the next
macroblock to be encoded is adjacent said present macroblock to be
encoded.
20. The system of claim 12, wherein said characteristic identifying
information comprises location information identifying the location
of at least one animation window in an image, wherein each of said
frames in said sequence is comprised of a plurality of multiple
pixel macroblocks, and wherein said encoder further includes means
for:
1) employing said location information to identify one or more
macroblocks in a frame to be encoded which include at least a
portion that forms part of said at least one animation window;
2) encoding said one or more macroblocks as intra-macroblocks;
and
3) encoding any remaining macroblocks in said frame to be encoded
as forward predictive coded macroblocks, with a zero motion
vector.
21. The system of claim 12, wherein said characteristic identifying
information comprises location information identifying the location
of at least one animation window in an image, wherein each of said
frames in said sequence is comprised of a plurality of multiple
pixel macroblocks, and wherein said encoder further includes means
for:
1) employing said location information to identify one or more
macroblocks in a frame to be encoded which include at least a
portion that forms part of said at least one animation window;
2) encoding said one or more macroblocks by employing a motion
vector search algorithm; and
3) encoding any remaining macroblocks in said frame to be encoded
as forward predictive coded macroblocks, with a zero motion
vector.
22. A method for encoding Internet Web page images using a video
encoding format, said method comprising the steps of:
a) providing an Internet browser application;
b) generating a sequence of Web page image frames with said browser
application;
c) providing said sequence of frames to a video image encoder;
d) providing additional information from said browser application
to said video image encoder, said information selected from the
group comprising scrolling coordinate information and animation
window location information; and
e) encoding at least one of said frames with said encoder by
employing said scrolling coordinate and/or animation window
location information, and information relating to a previous frame
in said sequence.
23. The method of claim 22, wherein said encoding is performed
using a digital video encoding format.
24. The method of claim 22, wherein said step of encoding further
comprises:
1) calculating at least one motion vector for a frame to be encoded
using said scrolling coordinate information; and
2) encoding said frame to be encoded as an inter frame using said
at least one motion vector and said image information for a
previous frame.
25. The method of claim 22, wherein said step of encoding further
comprises encoding a portion of said frame containing an animation
window as an intra frame.
26. A system for encoding Internet Web pages using a video encoding
format, said system comprising:
a) an Internet browser application for generating a sequence of Web
page image frames to be encoded, said browser application also
generating scrolling coordinate and/or animation window location
information relating to said sequence; and
b) an encoder for receiving said sequence of image frames from said
browser application and encoding said frames, said encoder
including means for obtaining said scrolling coordinate and/or
animation window information from said browser application, and
encoding at least one frame in said sequence by employing said
scrolling coordinate and/or animation window information and image
information relating to at least one previous image frame in said
sequence.
27. The system of claim 26, wherein said encoder is a digital video
encoder.
28. The system of claim 26, wherein said browser application and
said encoder are disposed in a cable television system headend, and
a plurality of downstream communication channels is provided for
transmitting encoded Internet Web page data from said encoder to
one or more remotely located set top converter boxes.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for
digitally encoding video image data, and is particularly suited for
encoding Internet Web pages for transmission and display.
With the ever-increasing popularity of the Internet, a number of
systems and devices have appeared in the marketplace that
substantially reduce the initial equipment expense required for
accessing the Internet. For example, inexpensive dedicated
processors are available which enable a user to access the Internet
using a telephone line, and download Internet Web pages for display
on the user's television set.
Recently, an even more attractive Internet access system has been
proposed which completely eliminates the need for a user to have a
telephone line and a dedicated processor running a browser
application locally at their premises. This system employs a
modified cable television (CATV) system that uses the downstream
cable channels to transmit Internet-based information to the system
users via for display on their television sets. Each user is
provided with a set top converter box that has been modified to
enable entry of data or commands via a keyboard, remote controller
or other input device. One or more upstream channels are provided
which transmit the entered data or commands to a headend server in
the CATV system. The headend server is interfaced to the Internet
via an Internet Service Provider (ISP), for example, and includes
processing equipment which can simultaneously operate a plurality
of resident Internet browser applications, one for each system user
requesting Internet access. The headend server therefore contains
all of the processing equipment necessary to access the Internet
through the ISP, while each user's set top box acts as an
input/output device for interfacing the user to the Internet.
In the operation of the system, a user requests Internet access by
entering an appropriate command into the set top box that transmits
the command through an upstream channel to the headend server. In
response, the headend server connects the user to one of the
resident browser applications via one of the system's downstream
channels.
The Internet-based information, e.g., Web pages, can be transmitted
through the downstream channel in a number of ways. In an analog
implementation, for example, the Internet data can be inserted into
the vertical or horizontal blanking intervals of the conventional
analog television signals which are simultaneously transmitted on
the selected downstream channel. In an all-digital embodiment,
however, the Internet data must be encoded in the same format that
is employed for digitally encoding video signals. More
particularly, the data must be encoded using standardized
procedures for encoding, storing, transporting and displaying
continuous video frames that have been specified by The Motion
Picture Experts Group (MPEG). Thus, the image bit map generated by
the browser application is not rendered at the headend, but instead
is further compressed by an MPEG image encoder. It is the
compressed image data that is transmitted to a user.
MPEG encoding is a video image compression technique that
substantially reduces the amount of motion picture image data that
must be transmitted. This data reduction is made possible because
spatial redundancy exists within an image frame (intra frame
compression). In addition, each succeeding frame in a motion
picture video usually contains substantial temporal redundancy,
i.e., portions which have either not changed from the previous
frame, or have only been moved relative to the previous frame
(inter frame compression). When spatial redundancy is removed from
a frame, the frame is said to be encoded as an intra-coded frame
(I-frame). In an inter frame compression scheme, two different
compression algorithms may be employed to generate two kinds of
encoded frames. A compressed image frame is called a
Predictive-coded frame (P-frame) if only a prior frame is compared
and the difference is coded. Another inter frame compression
results in a Bidirectionally predictive-coded frame (B-frame) if
both a prior frame and a post frame are used for encoding. In these
cases, it is not necessary to transmit all of the image data for
each frame. Instead, only the difference data representing the
portions in the current frame that have changed from the
neighboring (previous or later) frame(s) is transmitted. For areas
in an image which have been moved relative to the previous frame,
it is possible to search for these areas, and then generate a
motion vector which instructs a receiving decoder to construct a
portion of the next image frame by moving a corresponding portion
in the previous image frame a specified displacement and direction.
To encode a sequence of video frames, the first frame is encoded as
an intra or I frame where information for all of the pixels in the
frame needs to be transmitted since no previous frame information
is available. The next frame in the sequence can then be encoded
either as an P (predictive) frame or a B (bi-directional
predictive-coded) frame which includes only the difference or
motion vector data resulting from the frame comparisons. P or B
frames can continue to be used for encoding the succeeding frames
in the sequence until a substantial change, such as a scene change,
occurs, thus necessitating formation of another I frame. In
practice, however, the encoder is programmed to encode I frames at
a constant rate, such as for every other N frames. The MPEG
encoding procedure thus compresses images by suppressing
statistical and subjective redundancy inter and intra frames. An
MPEG decoder is capable of decompressing the coded image close to
its original format so that the decompressed image may be displayed
on a display device, such as a television or computer monitor.
In the Internet Web page display application, only P frames are
usually employed for inter frame compression because B frame coding
requires comparison with post (later in time) frames which are not
available immediately. However, a B frame can be encoded by forward
comparison only between the current frame and the prior frame as a
special case, and in this instance, can also be employed for Web
page inter frame compression.
In the application of MPEG encoding to the previously described
CATV system, each user's set top box includes an MPEG decoder for
decoding the digital video bit stream received on the downstream
channels. This requires that any Internet Web page image data to be
transmitted to the set top boxes also be MPEG encoded. An MPEG
encoder is thus incorporated in the cable headend to encode the
browser generated Web page image data, which usually is a bit map,
before it is transmitted on one of the downstream channels to a
user's set top box.
In general, however, MPEG encoding of Web page image data is
needlessly intensive from a computation standpoint since Web pages
do not usually incorporate full motion video, and often appear to
be nothing more than a still image. Strictly speaking, though, the
Web page is not a still image. Due to the limited viewing size of a
display device, the Web page is usually larger than the display
device's viewing area. A user may therefore scroll a Web page to
move the page horizontally or vertically to view the whole page.
Depending on the speed at which the page is scrolling, the images
on the display device may thus be considered to be a series of
video frames displayed at a variable frame rate. Other Web pages
may contain a small animation window in which several localized
pictures are alternatively displayed at a certain rate. JAVA
applets animation and regional character updates which occur as a
user types an e-mail message are other examples of this local
animation scenario. In both of these cases, MPEG inter frames may
be constructed after the generation of a first, intra frame, to
reduce the number of bits needed to represent each frame, thus
substantially reducing the required bandwidth in the communication
link.
As discussed previously, when an inter frame is generated, motion
vectors must be found, coded and transmitted so that the MPEG
decoder can reform the frame. A motion vector search is one of the
most difficult tasks in designing an MPEG encoder. Since the MPEG
committee defined only the syntax and semantics of a compressed
frame, but did not define how motion vectors searching should be
implemented, numerous proprietary motion vector search algorithms
were developed by various encoder vendors. For continuous video
compression, however, a motion vector search is very complicated
and requires a large percentage of the entire encoding
computational effort. More particularly, in MPEG encoding, each
video frame to be encoded is subdivided into a plurality of
multiple 64 (8.times.8) pixel blocks, and four such blocks covering
a 16.times.16 pixel area are known as a macroblock. During
encoding, the MPEG encoder searches for the best match between each
macroblock of a present frame to be encoded with the corresponding
macroblock in the previous frame. This search for the best match is
known as motion estimation.
The existing algorithms for motion estimation fall into two
categories: feature/region matching and gradient-based. In the
first category, both block matching and hierarchical block catching
can be employed for motion estimation. For encoding a continuous
video, the encoder has to search the entire screen (exhaustive
search) to find the best match because the encoder knows nothing
about the motion from frame to frame. In gradient-based motion
estimation, the exhaustive search may be avoided at the price of
solving linear equations during search.
All of the algorithms require many iterations to complete the
motion estimation. After the best match is found, the difference
between the matched macroblocks is calculated by comparing the
macroblocks. If the difference is small enough, a motion vector is
generated which determines the direction and offset of the motion.
Both the difference and the motion vector are encoded and
transmitted. If the difference is larger than a threshold, the
macroblock of the present frame is allowed to be intra compressed
as one encoded in an I frame.
In view of the foregoing, any video image encoding technique that
eliminates the need for motion vector search algorithms would be
desirable in view of the resulting substantial savings in
computation time and intensity.
SUMMARY OF THE INVENTION
The present invention provides an encoding technique for encoding
low-frame rate video image data, such as Internet Web pages, in
which motion vectors are generated without search algorithms by
taking advantage of prior knowledge regarding one or more
characteristics of the images. In the preferred embodiments of the
invention, the image characteristics are provided to an encoder,
such as an MPEG encoder, from an image generating application, and
relate to movement of or in the images.
More particularly, both embodiments of the invention are designed
specifically for use with CATV systems, as discussed previously,
which include Internet access capabilities. In these systems, when
a user scrolls through a Web page, scrolling input signals are sent
by the user's set top box to the browser application in the
headend. These signals define the direction of the scrolling and
its offset, typically in terms of x and y coordinates. In addition,
the Web pages may contain one or more animation windows, the
graphical content of which alternates or changes every second or
so. The browser application can easily detect whether one or more
animation windows is present in the Web page image, and if so,
determine the coordinates of the animation window(s). The scrolling
coordinate and animation window information can also be employed by
the encoder to determine the exact change between a previous image
frame and a present image frame that has occurred as a result of
the scrolling and/or animation window movement. With this
knowledge, a motion vector search is unnecessary, and can be
replaced with a set of calculations employing the scrolling
coordinates.
In the first preferred embodiment of the present invention, the
encoder employs the scrolling coordinates to determine motion
estimation for all of the macroblocks in the present frame relative
to the previous frame in a single step, and without a multiple
iteration search. A comparison between the macroblock of the
present frame and the corresponding macroblock of the previous
frame determined by the motion estimation, indicates whether the
changed macroblock is the same as the corresponding macroblock in
the previous frame which has been shifted in the direction and
amount specified by the scrolling coordinates. If so, the motion
vector for this macroblock of the frame has been located, and the
motion vector and the difference between the macroblocks is encoded
and transmitted. The process is repeated for each macroblock in the
frame to generate the resulting inter frame. The resulting motion
vector calculation and algorithm using the scrolling coordinates
requires much less computation than a full search algorithm.
In the second preferred embodiment, the encoder receives animation
window or other information from the browser application that
indicates that certain portions of an image are continuously
changing, and thus should be encoded as an intra frame. If the
browser application detects that one or more animation windows are
present in the Web page image, it determines the coordinates of the
animation window(s), and passes the coordinates to the encoder. The
encoder knows that only the portions of the Web page enclosed by
the animation window will undergo changes from frame to frame,
absent any scrolling operations. Thus, if the encoder receives
animation window coordinates from the browser application, the
encoder knows that it can encode the present frame of the Web page
by encoding only those macroblocks that are contained in the one or
more animation windows. These are encoded either as intra
macroblocks (no need for motion estimation) or as forward
predictive coded macroblocks by performing a motion estimation
constrained within the animation window. The remaining macroblocks
are encoded as zero motion vector blocks, which means that they
have not changed from the previous frame.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become
apparent from the following detailed consideration of a number of
preferred embodiments thereof, taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a block diagram of a CATV system which incorporates
Internet access capabilities, and can employ a digital encoding or
compression algorithm based on the principles of the present
invention;
FIG. 2 is an illustration of an Internet Web page having an
animation window therein;
FIG. 3 is a flow chart of an algorithm for locating portions of an
image frame which have moved relative to a previous image frame as
a result of a scrolling operation, said algorithm comprising a
first preferred embodiment of the present invention; and
FIG. 4 is a flow chart of an algorithm comprising a second
preferred embodiment of the present invention for encoding video
images having animation windows therein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a general block diagram of a CATV system 10 which
incorporates elements for facilitating access to the Internet by a
plurality of system users, and is illustrative of one type of
system with which the concepts of the present invention may be
employed. It should be noted that the CATV system 10 is illustrated
in general form since many of its detailed elements are not
necessary for an understanding of the present invention.
The CATV system 10 includes a cable headend 12 and a cable
television distribution network 14 for interfacing the headend 12
to a plurality of set top converter boxes 16. A plurality of
bi-directional transmission links 17 interconnects the set top
converter boxes 16 with the distribution network 14, each of which
includes a plurality of downstream channels 18 and one or more
upstream channels 19. For clarity, the details of only one of the
set top boxes 16 and associated elements are illustrated in FIG.
1.
The cable headend 12 receives video programming and Internet-based
information from remote sources (not shown), and transmits the
video programming and other information through the distribution
network 14 to the set top boxes 16. Typically, the video
programming is received from the remote source in either an analog
format, or a digitally compressed or encoded format, such as MPEG 1
or MPEG 2. The Internet-based information, on the other hand, is
typically HTML coded Web pages along with still images coded in
JPEG or GIF formats which is employed by one or more browser
applications 20 to generate Web page bit map images. A video
encoder 22, which may be an MPEG 1 or MPEG 2 encoder, for example,
is also provided in the headend 12 for encoding the Web page bit
map images before they are transmitted by the distribution network
14 to the set top boxes 16. The encoder 22 is employed to reduce
the bandwidth necessary to transmit the data generated by the
browser application 20, and to generate an MPEG standard compliant
video stream.
Each of the set top boxes 16 is interfaced via a terminal processor
24 and associated communication links 25 (e.g., cables, infrared
wireless links, etc.) to a television or monitor 26, and one or
more input devices, such as a wireless keyboard 28 and a remote
controller 30. As each set top box 16 receives the digitally (e.g.,
MPEG) encoded or compressed video programming and Internet-based
information from the distribution network 14, it is passed through
a decoder 32 which restores the video programming signals and Web
page image data to their original form for display on the
television or monitor 26. The decoder 32 is of the same format as
the encoder 22, such as MPEG 1 or MPEG 2, for example.
The CATV system 10 thus allows a system user to conduct an Internet
session by sending appropriate commands via the keyboard 28 and/or
remote controller 30 to the headend 12. In response, the headend 12
connects the user to one of the browser applications 20, and
retrieves the requested Internet information from the remote
source. The visual information generated by the browser application
20 is encoded and downloaded to the user's set top box 16 for
display on their television or monitor 26.
To encode the Web page image bit map data generated by the browser
application 20, the encoder 22 employs any known conventional
algorithm for encoding motion picture video frames, such as MPEG 1
or MPEG 2. Motion picture video encoders encode and compress the
video data by comparing each successive video frame with a previous
frame, and transmitting a predictive-coded or P frame which
includes information only about pixels in the video image that have
changed from the previous frame. In motion picture video, where
each succeeding frame usually differs only slightly from a previous
frame, this technique results in a substantial reduction in the
amount of data that has to be transmitted with each frame. A P
frame primarily consists of motion vectors and prediction errors
associated with each macroblock. The motion vector of a macroblock
indicates the direction and magnitude of the motion of the
macroblock from the previous frame to the present one. All motion
vectors are coded using variable length coding (VLC). The
prediction errors identify the difference between the macroblock in
the previous frame and the predictive one in the present frame
shifted by the motion vector. The prediction errors are transformed
into frequency domain by using a Discrete Cosine Transform whose
coefficients then are coded by VLC. This technique also results in
a substantial reduction in the amount of data that needs to be
transmitted for each image, especially in situations where large
portions of a total image are moving from frame to frame.
Since most existing motion vector search algorithms are very
iterative in nature, they are computationally intensive and time
consuming to employ. The present invention eliminates the need for
employing motion vector search algorithms in specific instances
which are particularly applicable to the encoding of the Web page
image data where additional information regarding any changes from
frame to frame in the image is available for use by the encoder 22.
With this additional information, the encoder 22 is able to
identify portions of a Web page in a present frame, which have
moved or changed from the previous frame, by using a set of
calculations instead of a motion vector search algorithm.
In a first preferred embodiment of the invention, a motion vector
to be employed in the formation of an MPEG P frame, is generated in
which the motion of a Web page or other image is a result of a
scrolling operation performed by an end user through use of their
keyboard 28 or remote controller 30. FIG. 2 illustrates a Web page
50 as displayed in a browser application display image 52. The Web
page 50 includes an animation window 53 that is a rectangular
shaped area in which the graphical content changes or alternates
periodically, such as once every second. As is conventional, the
browser application provides a number of scrolling buttons 54 along
the right side of the display image 52 to facilitate scrolling
operations using the remote controller 30 as a pointing device. In
the example of the system 10 of FIG. 1, when a user scrolls the
displayed Web page 50 using either the keyboard 28 or the remote
controller 30, the scrolling commands are sent to the browser
application 20 which performs the actual scrolling operation. This
translates the Web page image, thus necessitating the formation of
a number of a P frames by the encoder 22 to transmit the necessary
image change information to the user's set top box 16.
FIG. 3 is a flow chart of the steps employed in the first
embodiment of the present invention to encode the Web page images
when a scrolling operation occurs. First, at step 100, when the
browser application 20 detects a scrolling operation, it will
notify the encoder 22 to form a P frame for the operation. In
addition to the operation type, the browser application 20 will
pass two scrolling parameters to the encoder 22, these being the
scrolling direction and offset or distance. The direction of the
scrolling is defined as follows:
(1) Horizontally scrolling left: the content in a window contained
in the Web page image moves right;
(2) Horizontally scrolling right: the content in a window contained
in the Web page image moves left;
(3) Vertically scrolling up: the content in a window contained in
the Web page image moves downward;
(4) Vertically scrolling down: the content in a window contained in
the Web page image moves upward; and
(5) Horizontally and vertically scrolling simultaneously: the
content in a window contained in the Web page image moves
diagonally.
As discussed previously, in an MPEG encoder, for example, two
consecutive frames of a video image, F(n-1) and F(n), are employed
for motion vector calculation. Suppose frame F(n) is a result of
scrolling from frame F(n-1). In this case, the scrolling direction
and scrolling offset or distance are determined by a pair of
scrolling parameters, Xs and Ys, which are illustrated in Table
1:
TABLE 1 Xs < 0 Xs = 0 Xs > 0 Ys < 0 Scrolling diagonally;
No horizontal Scrolling diagonally; upward vertically by scrolling;
upward vertically by .cndot.Ys.cndot. pixels and left scrolling
upward .cndot.Ys.cndot. pixels and right horizontally by
.cndot.Xs.cndot. vertically by .cndot.Ys.cndot. horizontally by Xs
pixels pixels pixels Ys = 0 Scrolling left No horizontal Scrolling
right horizontally by .cndot.Xs.cndot. scrolling; no horizontally
by Xs pixels, no vertical vertical scrolling pixels, no vertical
scrolling scrolling Ys > 0 Scrolling diagonally; No horizontal
Scrolling diagonally; downward vertically by scrolling; downward
vertically by Ys pixels and left scrolling downward Ys pixels and
right horizontally by .cndot.Xs.cndot. vertically by Ys
horizontally by Xs pixels pixels pixels
Usually, a Web page scrolling does not require full screen
scrolling because, (1) an overall fixed window frame exists all of
the time which does not scroll at all; and (2) there are some Web
pages containing multiple sub-windows. The contents in the
sub-windows may be scrolled separately and independently. It is
assumed that scrolling may move the contents only in one of the
sub-windows. The display screen, therefore, is divided into a
scrolling area and a non-scrolling area.
With reference again to the flow chart of FIG. 3, the next steps
are employed to identify scrolling and non-scrolling areas of a Web
page or other image by dividing an image frame into a plurality of
multiple pixel macroblocks (MB). For example, each macroblock can
be a square 16.times.16 pixels in size. Suppose that a frame, F(n),
consists of M rows and N columns of macroblocks:
and
where p.sub.u,v (n) is a pixel at coordinate (u, v) in the nth
frame.
The motion vectors are calculated in the following manner. Each
macroblock in the present frame is examined one at a time. For
example, the process is started with the macroblock on the left top
corner of the frame, i.e., i=0 and j=0.
First, at step 102, the macroblock in the present (nth) frame is
compared with the macroblock in the same location in the previous
or (n-1)th frame to determine if the difference in pixel
intensities between the two macroblocks is smaller than a
predetermined threshold Th as determined by equation 2:
##EQU1##
If the inequality in equation 2 is satisfied, the macroblock
MB.sub.i,j is considered to be in the non-scrolling area of the
image. In other words, the macroblock has not changed from the
(n-1)th frame to the nth frame. Thus, the motion vector associated
with this macroblock is set to zero at step 103. That is,
If, on the other hand, the inequality in equation 2 is not
satisfied, then the algorithm proceeds to step 104 in which the
macroblock in the nth frame is compared with the same macroblock
shifted by either Xs or Ys in the (n-1)th frame by using the
following equation: ##EQU2##
If the inequality in equation 4 is satisfied, the macroblock
MB.sub.i,j (n) is said to be in the scrolling area, and can be
reconstructed at step 106 by shifting macroblock
MB.sub.i+X.sub..sub.s .sub./16,j+Y.sub..sub.s .sub./16 (n-1) by -Xs
and -Ys pixels. Notice that the shifting offsets, X.sub.s and
Y.sub.s, may not necessarily be multiple of integer 16, therefore,
the macroblock may not align with macroblock boundary. It follows
from the definition of a motion vector that the compensation motion
vector for this macroblock MB.sub.i,j (n) is:
If both of the inequalities in equations 2 and 4 are not satisfied,
the macroblock MB.sub.i,j (n) cannot be identified using a motion
vector, and must be encoded at step 108 as an intra-macroblock.
Once either a motion vector is generated for the macroblock using
equations 3 or 5, or the macroblock is encoded as an
intra-macroblock, the algorithm proceeds to step 110 in which the
indices, i and j, are incremented, and are checked to see whether
the last macroblock in the frame has been encoded. If not, the
program returns to step 102 to encode the next macroblock in the
frame. If the last macroblock has already been encoded, the
encoding of the P frame is complete, and the procedure is repeated
for the next frame.
The foregoing motion vector calculation algorithm requires much
less computation than a full screen search algorithm. Furthermore,
the best case will be a found match between two macroblocks in the
first step 102 of the procedure. In this case, the search for the
motion vector is done in one step for the macroblock. It is assumed
that the neighboring macroblocks will fall in the same kind of area
(scrolling or non-scrolling). If a macroblock is found in a
scrolling area, then for the next macroblock on the same slice,
step 102 may be switched with step 104 in the process to check
whether the macroblock is in the scrolling area first, since it is
likely that this is the case. In other words, the conditional
probability that a macroblock MB.sub.i+1,j (n) will fall in a
scrolling area, given that the adjacent macroblock, MB.sub.i,j (n)
is in the scrolling area, is much higher than the likelihood that
the macroblock MB.sub.i+1,j (n) is in a non-scrolling area as
defined by the following equation:
where S is a scrolling area. Similarly, if a macroblock is found in
a non-scrolling area, no switch between step 102 and step 104 a is
necessary for checking the next macroblock for the same reason.
Therefore, most of the macroblocks need only one step to find their
motion vectors. Of course, the worst case scenario will be when no
matches are found in either step 102 or step 104. However, this
should rarely happen in a normal scrolling operation, with the
exception of the case where both scrolling and animation occur
simultaneously.
Turning now to a second preferred embodiment of the invention, FIG.
4 is a flow chart illustrating the steps carried out by an
algorithm which detects an animation window in a Web page, and
encodes the Web page image data in accordance with the animation
window coordinates. Returning briefly to FIG. 2, the animation
window 53 can be defined by x and y coordinates. Let Sx and Sy be
the total number of pixels in the x and y directions, respectively,
of the entire browser application display image 52. The animation
window 53 can then be defined as:
With reference to FIG. 4, a first step 200 in this embodiment of
the invention is for the browser application 20 to detect whether
one or more animation windows is/are present in the currently
displayed Web page. If so, the browser application 20 passes, at
step 202, the x,y coordinates of the one or more animation windows
to the encoder 22.
Next, at step 204, the encoder 22 determines whether the animation
window coordinates align with the macroblock boundaries employed by
the encoder 22. In the vast majority of cases, the animation
window(s) will not line up with the macroblock boundaries. In this
case, the encoder 22 proceeds to step 206 to adjust the macroblock
boundary alignment so that each animation window is expanded to its
nearest macroblock boundaries. The following equations are used for
the macroblock boundary alignment adjustment: ##EQU3##
The four equalities of equation 8 are integer operations which
generate a new set of coordinates that define a new animation
window whose frame boundary aligns with the macroblock boundaries,
and is defined as:
Since all of the macroblocks in an image frame are mutually
exclusive (no overlapping), the animation window in equation (9)
can be redefined as:
Equation 10 defines a window that consists of all of the
macroblocks enclosed by the window. This equation is therefore used
at step 208 to verify whether a particular macroblock belongs to
the animation window or not.
Once the macroblocks defining the one or more animation windows
have been determined at step 208 using the foregoing equations, the
next step 210 is to encode the Web page image data as a P frame.
For a given animation window of a Web page, the following substeps
of step 210 are employed to encode the P frame. At step 212, the
algorithm starts with the top left macroblock in a frame (i=0 and
j=0) and continues the following algorithm for all of the
macroblocks in the frame. If a macroblock MB.sub.i,j (n) is found
by equation 10 to be an element in an animation window, then two
options are available to encode the macroblock. If encoding time is
a major consideration, it will be simply encoded at step 214 as an
intra macroblock at the price of more bits generated. If higher
compression ratio is more preferable due to communication bandwidth
limitation, any motion estimation algorithm may be employed at step
215 to search for motion vectors within the animation window. This
process is not nearly as intensive as performing a motion vector
search of the full screen image since the animation window is much
smaller than a full screen. If a macroblock MB.sub.i,j (n) is not
located within the animation window, it is encoded at step 216 as a
forward compensated macroblock, with a zero motion vector.
Basically, this macroblock can be skipped if it is not at the very
beginning or the end of a slice. In other words, this macroblock
will not have changed from the previously encoded frame, and thus
no new information need be encoded for this macroblock. Thus, only
those macroblocks which are included in any detected animation
windows that are present in the Web page will need to be encoded by
the encoder 22, and transmitted to the set top converter box's
decoder 32. Finally, at step 218, the next macroblock, if any, in
the frame is selected, and encoded by returning to step 212.
In a case where both scrolling and animation occur simultaneously
when a user scrolls a Web page with animation window(s), the
browser application must pass a scrolling operation type to the
encoder. The inequalities of both equations (2) and (4) in the
first preferred embodiment of the invention will not be satisfied
for the macroblocks in an animation window during scrolling. Those
macroblocks will be automatically encoded as intra macroblocks.
In conclusion, both embodiments of the present invention provide
simplified video encoding or compression for encoding image data,
particularly Web page images. In both embodiments of the present
invention, knowledge of prior operation conditions completely
eliminates the need to perform motion vector searches during
encoding of a P frame. Instead, the prior knowledge enables the
motion vectors to either be calculated by given equations, or set
to zeros. This efficient technique makes it feasible to create an
MPEG encoder, for example, in a real time software program.
Although the invention has been disclosed in terms of a number of
preferred embodiments, it will be understood that numerous
variations and modifications could be made thereto without
departing from the scope of the invention as defined in the
following claims. For example, although the preferred embodiments
are directed specifically to encoding of Internet Web pages, the
invention can obviously be applied to any video or image encoding
application where prior knowledge of the changes or movement in the
video or images is available so that motion vector searches need
not be utilized to generate P and B frames. In addition, the
invention is not limited to use with MPEG encoders for generating P
frames, and can be employed with other video encoding schemes
and/or encoded frame types.
* * * * *